1. Field of the Invention
The invention relates to plasticizer-containing films based on polyvinyl acetal having a selective permeability for UV radiation and a low tendency to yellow, and to laminated glass produced using these films.
2. Background Art
Plasticizer-containing polyvinyl butyral film (PVB film) of the type usually found on the market for laminated glass has for many decades been furnished with UV absorbers in order to prevent UV radiation from penetrating the laminated glass in contrast with visible light. In this way, almost complete protection from harmful UV radiation can be ensured for persons or objects behind the laminated glass in a simple manner. In addition to protection from cutting injuries, this has the important advantage over monolithic glass that it is largely transparent for UV radiation.
For the use of PVB film as an encapsulating material in photovoltaic modules, increased quantities of UV absorbers are used because it is assumed that due to the intense sunlight exposure of solar modules, yellowing of the PVB film might otherwise occur as a result.
Derivatives of 2-hydroxyphenylbenzotriazole, which are available with various substitution patterns, e.g., under the brand names Tinuvin P, Tinuvin 328, Tinuvin 327, Tinuvin 326, have proven especially successful as UV absorbers. UV absorbers of the benzotriazole type in PVB film are known for their excellent long-term stability, good solubility and high absorption performance in the entire UV range (UV-B and UV-A). Depending on the substitution pattern, however, absorption also occurs more or less strongly also in the visible range of the spectrum, e.g., at 400 nm, so these UV absorbers may cause a certain yellow discoloration of the PVB film, depending on the quantity used and the substituent distribution.
If the most thorough possible protection from UV radiation—i.e., more than 99% absorption—is required for the predominant number of possible applications of laminated safety glass, then there are special applications in which a high permeability for UV radiation would be desirable:                in greenhouses for growing vegetables or cut flowers, the highest possible energy input and a minimum of UV radiation are required. In addition, the insects used for pollination (bees, bumble-bees) are also oriented based on UV contrasts.        in photovoltaic modules with cell types which also use portions of the UV spectrum for generating a photoelectric current and in which the PVB film is arranged in front of them on the side facing the light, the energy efficiency turns out to be lower if the PVB film is almost completely opaque for UV radiation when using conventional UV absorbers of the benzotriazole type.        in applications in the indoor area, where no UV protection is required, an increased UV transmission is allowed, to make colorless elements appear to be more natural in color, in particular in combination with low-iron glass, because UV-stabilized PVB film has a low yellow value of the composite.        as a UV-permeable laminated glass for assembly with UV-A-curing adhesives, e.g., in the furniture glass field.        as part of bird-friendly glazing (i.e., for protection of birds), in which one pane of glass laminated to the PVB film should be partially UV-permeable to achieve a contrast that can be perceived by birds.        